Use gdb_bfd_sections in restore_command

Use gdb_bfd_sections in restore_command

This changes restore_command to avoid bfd_map_over_sections, in favor
of iteration.  A helper data structure can also be removed by this
patch.

gdb/ChangeLog
2020-09-19  Tom Tromey  <tom@tromey.com>

	* cli/cli-dump.c (struct callback_data): Remove.
	(restore_one_section): Rename from restore_section_callback.
	Change parameters.
	(restore_binary_file): Change parameters.
	(restore_command): Use foreach.